I thought he looked stupid as all **** at first, because I was expecting just a normal evil king who had turned evil for an understandable reason. Nightmare Moon wanted recognition, and her design reflected her status as princess of the night. Chrysalis was hungry, and her design represented her parasitic nature. Discord represents chaos, it's his job, so his design represented chaos.

Sombra on the other hand was just totally ******* bonkers. He was kind of like Sauron, from Lord of the Rings. Once I saw him in action, how vicious and corrupt he was, unlike the other three main villains, his design made a lot more sense to me.

( Warning: Bad English)
I didn't think, he were bad.
look at it this way...
Remember, that Sombra had the ''Shadow magic'' eyes like Twilight and Celestia, but unlike them, his were permanent.
I believe, that the ''Shadow magic'', is a spell, that corrupt things and turn them into shadows of their previous self, a perfect tool for an emperor, that have lost his people trust. So in return for a people in submission, he turned them all into shadows, without free will and without memories. But this could come with a price.
The corruption may have affected Sombra too, what if the corruption took away his humanity and memories too, so he also only were a shadow of his previous self. Remember, that Celestia and Luna destroyed his physical form and only left him with his tainted soul, which only purpose were to ''reclaim his slaves'', when he returned. He had no intellect and no reasons. He were simply a beast, which instincts told him what to do, remember, that he could barely form a sentence.
Bottom line is, he had no background story, because he didn't have any reason to talk about it, he didn't have any reason for his actions at all.

The Sombra we saw in the premier was only a fragment of what Sombra used to be.

Sombra's use of the dark magics is what made him evil to begin with. It tainted his soul further and further until there was nothing good left in it. he used the dark magic to retain his control over the Crystal Empire after its citizens finally decided he was a terrible king and should be removed.

Before his defeat, he was a rational being with an evil soul. Celestia and Luna destroyed his physical presence, along with the rational thought and cognitive functions that the physical brain would have provided him. What we saw in the premier was merely the evil soul, what drove his physical self to begin with. Effectively a braindead lunatic, so to speak.

He was driven to retake the Empire and re-enslave everyone because that's what he was doing when he was destroyed. He's a ghost re-living his past. The ghost doesn't know why it's doing it, and maybe it can't even think at all. It's just a ghost doing what it's doing.